That's on the registry, not the registrar. Point your anger at whoever owns that TLD.
If we assume that Namecheap is incapable of malice, then it means Namecheap's systems and/or processes are broken. (Registry agreements don't allow registries to retroactively reclassify domains as "premium".)
On what do you base this? Registration agreements allow quite a lot in favor of registrars and registries.